1. Field of the Invention
The present invention relates to an antenna applied to a small portable terminal in which dimensions of a terminal such as a portable mobile terminal are under 1/4 of a wavelength.
2. Description of the Related Art
In a small antenna or a built-in antenna applied to a small terminal having a small volume such as a portable mobile terminal, impedance matching with a radio frequency becomes more difficult as the antenna becomes smaller in size and becomes to be formed in a built-in structure, thus requiring various measures. In a related art, miniaturization is aimed at with a linear-plate inverse-F antenna that is excellent in point of miniaturization and a built-in structure as a basis. The details thereof have been discussed in JP-A-2-22563 and JP-A-2-21164 for instance.
In all of the related art, matching is obtained with inverse-F antennas presenting capacitive characteristics essentially by using a matching pin presenting inductive characteristics provided in the vicinity of an exciting point.
A perspective view and a side view of a typical inverse-F antenna are shown in FIG. 22 and FIG. 23. The inverse-F antenna can be regarded as a distributed constant circuit formed of a capacitive component C.sub.2 formed between a horizontal portion of a laminar antenna 27 and a finite ground, a capacitive component C.sub.1 formed between a feed line 2a and a vertical portion (a matching pin) of the laminar antenna 27 and an inductance L proportional to a height l of the vertical portion (the matching pin) of the laminar antenna 27 when viewed from an exciting point 4. Since the capacitive component C.sub.2 is increased as the dimensions of the antenna get smaller in a height direction in the case of miniaturization of an antenna, it is required to increase the component of inductance L in order to deny the increase of C.sub.2. However, the height l of the vertical portion (the matching pin) of the laminar antenna 27 is also decreased as the antenna dimensions get smaller in the height direction and the value of the inductance L gets smaller. Therefore, a method of increasing the inductance L equivalently by using an inductive electromagnetic field is adopted in the related art. In general, an electromagnetic field includes components that attenuate in proportion to the first power, the second power and the third power of the distance, but the component that attenuates in proportion to the second power of the distance is the inductive electromagnetic field. Since the electromagnetic field that attenuates in proportion to the second power of the distance is used in order to increase the inductance L, it becomes required to make the distance d between the feed line 2a and the vertical portion (the matching pin) of the laminar antenna 27 smaller. When the distance d is made too small, however, the influence by an electromagnetic wave that attenuates in proportion to the third power of the distance is also exerted at the same time. Since the electromagnetic wave that attenuates in proportion to the third power of the distance increases an ohmic loss of a conductor forming an antenna, the loss of a feed line or a matching pin that is a conductor is increased, thus lowering antenna efficiency. Therefore, there has been a subject of solving such a problem.